The present invention relates to rocket propulsion systems.
There are two basic types of chemical rocket propulsion systems in wide use today, one type using liquid propellant and another type using solid propellant. A third rocket propulsion system, considered an alternative to liquid and solid propellant rocket propulsion systems, is called a hybrid propulsion system. In a hybrid propulsion system, one component is stored in the solid phase, while another component is stored in either the liquid or gaseous phase. In most hybrid propulsion systems, the solid component is the fuel, and the liquid or gas component is the oxidizer.
Rockets using hybrid propulsion systems offer certain advantages over rockets using solid or liquid systems. For example, using a hybrid propulsion system for a rocket allows for thrust termination, rocket motor restart, and throttling capabilities. Furthermore, hybrid propulsion systems are inherently immune to explosion. Immunity to explosion is of great importance to rocket-powered vehicle designers that hope one day to operate their sub-orbital and orbital spaceplanes alongside jet-powered vehicles at public use airports. The safety and simplicity of the hybrid propulsion systems leads to lower development costs for new systems and lower operational costs. Additionally, the combustion products are generally very benign, producing a lower environmental impact. Finally, rockets with hybrid propulsion systems typically have a less complex design with a potentially higher reliability, and are also comparably less costly to develop, manufacture, and operate than the other rockets. The solid fuel grain of a hybrid rocket is typically formed by either casting melted sold fuel within a fuel motor case or by separately casting the fuel grain in a mold. In the latter instance, the solid fuel grain is “cartridge-loaded” into a motor case after the molding process. In this manufacturing method, the nozzle is engineered to be separated from the case to allow such loading. In another variation of this method, the solid fuel grain may be separately molded in multiple sections, allowing easier handling.
In a hybrid propulsion system, the burning rate is limited by the heat transfer from the relatively remote flame to the burning surface of the fuel. One of the physical phenomena that limits the burning rate in the hybrid propulsion system is the so-called blocking effect that is caused by the high velocity injection of vaporizing fuel into the gas stream. This difference in the combustion scheme of the hybrid propulsion systems significantly alters the burning rate characteristics compared to a solid propellant. Consequently, the regression rate of a conventional hybrid fuel is typically one-tenth or less than that of a solid propellant.
For a given selection of fuels and oxidizer-to-fuel mass ratios, the thrust generated by a rocket is approximately proportional to the mass flow rate. For a rocket with a hybrid propulsion system using a slow-burning conventional fuel, high thrust can only be achieved by increasing the burning surface area. The high burning area requirements and design constraints (such as the maximum grain length-to-port diameter ratio), lead to complicated multi-port configurations, such as a wagon wheel configuration. The complicated configurations cause further inefficiencies, such as unburned fuel.
The low regression rates and consequent multi-port design requirements often make rockets with a hybrid propulsion system an unattractive option, even though they offer significant advantages over current liquid and solid systems. In order for the hybrid propulsion systems to find use as a practical design with a variety of applications, higher regression rates are required. Therefore, it would be highly desirable to provide a propellant and fuel grain architecture that exhibits a high regression rate, without compromising the safety or the lower manufacturing cost associated with hybrid propulsion systems.